Process for preparing ceramic powder

ABSTRACT

The process for preparing ceramic powder having a core-shell structure by dissolving a crystalline ceramic powder in water so as to leave a core portion of the ceramic powder while heating; depositing a component of the ceramic powder dissolved in the water on and around surfaces of the core portion thereof as a deposited material from the water by gradually cooling the ceramic powder and the water obtained in the step of dissolving; and firing a mixture of the ceramic powder obtained in the step of depositing with an additive at a high temperature to subject the deposited material and the additive to solid phase reaction to form a shell portion on and around the core portion of the ceramic powder. The process can produce the ceramic powder having a core-shell structure from a crystalline ceramic powder, thereby achieving high electrical and mechanical features when formed into ceramic electronic parts. 
     On the other hand, conventional ceramic electronic parts produced from highly crystalline ceramic powder raise the manufacturing cost because such ceramic powder is low in reactivity so that it requires high firing temperature. Ceramic powder in a core-shell structure produced from low-crystalline ceramic powder results to conventional ceramic electronic parts which are poor in electrical and mechanical characteristics.

CROSS-REFERENCE

This application claims the priority of Japanese Patent Application No.H9-219982, filed on Jul. 31, 1997, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing ceramic powderand, more particularly, to a process for preparing ceramic powder in acore-shell structure.

2. Description of the Related Art

As ceramic electronic parts have recently been developed so as to maketheir sizes smaller and smaller and to make their quality higher andhigher, a variety of materials have also been developed so as to catchup with such development. Under such technical background, ceramicpowder having a core-shell structure is now being highlighted as amaterial for ceramic electronic parts.

Such ceramic powder having a core-shell structure is configured in sucha manner that it comprises a highly crystalline core portion and a shellportion formed on and around the core portion and composed of adifferent composition material. The such ceramic powder can smoothtemperature features of a ceramic condenser when it is used as adielectric material.

The ceramic powder with such a core-shell structure can be produced, forinstance, by admixing ceramic powder with an additive and firing theresulting mixture at a high temperature to thereby subject it to solidphase reaction to allow the additive to be formed on and around surfacesof the ceramic powder.

Further, it is favorable if ceramic powder having a core-shell structurecould be produced from highly crystalline ceramic powder because such ahighly crystalline ceramic powder has good electrical features such asdielectric constant and so on. The highly crystalline ceramic powder,however, has the drawbacks that it is so low in reactivity that aconsiderably high temperature is required to cause an occurrence ofsolid phase reaction, resulting to a rise in manufacturing cost.

On the other hand, a low-crystalline ceramic powder can produce ceramicpowder with a core-shell structure because it can readily be subjectedto solid phase reaction by firing it at a lower temperature. Such alow-crystalline ceramic powder, however, presents the drawbacks thatceramic electronic parts prepared therefrom may result in poorelectrical and mechanical characteristics.

SUMMARY OF THE INVENTION

Therefore, the present invention has the object to provide a process forpreparing ceramic powder with a core-shell structure having goodelectrical and mechanical features by firing ceramic powder at a lowertemperature.

In order to achieve the object, the present invention provides a processfor preparing ceramic powder, which comprises the step of dissolving acrystalline ceramic powder in water so as to leave a core portion of theceramic powder while heating; the step of depositing a component of theceramic powder dissolved in the water on and around surfaces of the coreportion thereof as a deposited material from the water by graduallycooling the ceramic powder and the water obtained in the step ofdissolving; and the step of firing a mixture of the ceramic powderobtained in the step of depositing with an additive at a hightemperature to subject the deposited material and the additive to solidphase reaction to form a shell portion on and around the core portion ofthe ceramic powder

Other objects, features and advantages of the present invention willbecome apparent in the course of the description which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process in accordance with the present invention can provide ceramicpowder having a core-shell structure by dissolving a crystalline ceramicpowder in water so as to leave a core portion of the ceramic powderwhile heating; depositing a component of the ceramic powder dissolved inthe water on and around surfaces of the core portion thereof as adeposited material from the water by gradually cooling the ceramicpowder and the water obtained in the step of dissolving; and firing amixture of the ceramic powder obtained in the step of depositing with anadditive at a high temperature to subject the deposited material and theadditive to solid phase reaction to form a shell portion on and aroundthe core portion of the ceramic powder.

The ceramic powder to used for the present invention may include, forexample, barium titanate powder, barium titanate zirconate powder, leadtitanate zirconate powder or lead titanate powder, although the ceramicpowder to be used for the present invention is not restricted to thosespecifically stated above.

As the crystalline ceramic powder to be used for the present invention,there may be preferably mentioned, for example, one synthesized byhydrothermal method. It is to be noted herein, however, that thecrystalline ceramic powder is not restricted to the one as stated aboveand any other ceramic powder produced by different methods may also beused as long as it is crystalline.

In accordance with the process of the present invention, it is preferredthat the ceramic powder and water are heated at temperature ranging fromapproximately 80° C. to 120° C. In this case, a speed of dissolving theceramic powder in water can be facilitated by heating the ceramic powderand water under elevated pressure. It is to be noted herein that theduration of heating the ceramic powder can exert the effect upon thethickness of the shell portion of the ceramic powder. When the ceramicpowder is heated for a longer time, on the one hand, the shell portioncan be made thicker. When the ceramic powder is heated for a shortertime, on the other, the thickness of the shell portion may becomethinner.

It is further preferred that the mixture of the ceramic powder withwater is gradually cooled at the rate of approximately 2° C. to 20° C.per hour. If the speed of gradually cooling the mixture at the rate oflower than 2° C., on the one hand, it will take a too long time to coolthe mixture. If the mixture is cooled at the rate of greater than 20°C., on the other, the component of the ceramic powder dissolved in thewater may not be deposited on and around surfaces of the core portionthereof as the deposited material to a sufficient extent. It is to benoted herein that, as the deposited material has a hydroxyl group, awater content or voids resulting from pores, the reactivity of thedeposited material with the additive is considered to be high.

Further, the additive to be used for the present invention maypreferably include, for example, Nb2O5, Co3O4, MnO2, Ta2O5 or VO5,although the additive is not restricted to those stated hereinabove andany other additive can also be used as long as it can achieve theeffects sought to be attained by the present invention.

The present invention will be described in more detail by way ofexamples.

EXAMPLE 1

Into an autoclave were placed 100 grams of highly crystalline bariumtitanate powder having an average particle size of 0.5 micron,synthesized by hydrothermal method, and 200 ml of purified water, andthe autoclave was closed in airtight way. The resulting mixture washeated at temperature ranging from 80° C. to 100° C. for 5 to 10 hours,as indicated as Sample Nos. A to D in Table below, thereby allowingsurfaces of particles of barium titanate powder to be dissolvedgradually in the purified water.

Then, the autoclave was gradually cooled to below 50° C. by lowering thetemperature of the autoclave at the rate of 2° C. to 20° C. per hour, asindicated in Table below.

To the resulting slurry of barium titanate powder obtained at 50° C. wasthen added 1.14 gram of Nb2O5 and the resulting mixture was mixed with 5mm φ ZrO2 beads for 1 hour.

The resulting slurry was then filtered yielding a cake that in turn wasdried at 110° C. for 24 hours, followed by firing it at 1,100° C. for 5hours in the atmosphere to give ceramic powder.

The resulting ceramic powder was then analyzed with XRD. As a result,the shift of a peak at 45.26° (CuK α) indicating the extent of solidsolution of Nb into barium titanate is as shown in Table below.

Further, the resulting ceramic powder was measured for its dielectricconstant and the results are shown as Sample Nos. A to D in Table below.

COMPARATIVE EXAMPLE 1

A mixture of 100 grams of highly crystalline barium titanate powderhaving an average particle size of 0.5 micron, as used in Example 1above, with 0.5 gram of Nb2O5 was admixed intact with each other andfired at 1,100° C. for 3 hours. The resulting ceramic powder was thenanalyzed with XRD. As a result, the shift of a peak was observed at45.26° (usual solid reaction) is as shown in Table below. Further, asshown in Table below, the resulting ceramic powder was found to have itsdielectric constant of 2,310.

                  TABLE                                                           ______________________________________                                                       Heating Heating                                                                             Lower-                                                                              Shift  Dielectric                              Temp. Hour ing Rate Of Constant                                             Sample Material (° C.) (hrs) (° C./hr) Peak ε         ______________________________________                                        Compara-                                                                             Barium  --      --    --    45.26°                                                                          2310                                tive Ex #1  titanate                                                            Ex-                                                                            A     Barium  80    5       2   45.26° ±                                                                   2813                                am-       titanate                                    0.19°                                                       ple     B  Barium       100                                                   5        2     45.26°                                                 ±  2903                          #1   titanate                                   0.24°                        C  Barium       80   10        2      45.26° ± 2755                                                               titanate                                                            0.22°                           D  Barium       80    5       20      45.26° ±  2844                                                              titanate                                                               0.07°                      ______________________________________                                    

From the results as shown in Table above, it is found that the ceramicpowder prepared in Example 1 has the thick shell portion formed on andaround the core portion having a high crystallinity when the extent ofthe shift of the peak at 45.26° (CuK α) taken into account, as comparedwith the ceramic powder obtained in Comparative Example 1.

Further, the ceramic powder obtained in Comparative Example 1 had thedielectric constant at 2,310 while the ceramic powder obtained inExample 1 had the dielectric constant in the range of from 2,755 to2,903. This means that the dielectric constant of the ceramic powderaccording to the present invention is considerably higher than that ofComparative Example 1.

EXAMPLES 3-5

The procedures were carried out in substantially the same manner as inExample 1 with the exception that barium titanate zirconate powder, leadtitanate zirconate powder or lead titanate powder was used,respectively, in place of barium titanate powder.

As a result, it was found that substantially the same results weregained as in Example 1 above.

EXAMPLES 6-8

The procedures were conducted in substantially the same manner as inExample 1 with the exception that MnO2, Ta2O5 or VO5, was used as anadditive, respectively, in place of Nb2O5.

As a result, it was found that substantially the same results weregained as in Example 1 above.

EFFECTS OF THE INVENTION

The present invention can provide the effects that the ceramic powderhaving an ideal core-shell structure, that is, the ceramic powder havinghigh electrical and mechanical features, particularly high temperaturefeatures, can be prepared at a relatively low temperature and at lowmanufacturing cost.

What is claimed is:
 1. A process for preparing ceramic powder,comprising:the step of dissolving a crystalline ceramic powder in waterso as to leave a core portion of the ceramic powder while heating; thestep of depositing a coating of the ceramic powder dissolved in thewater on and around surfaces of the core portion thereof as a depositedmaterial from the water by gradually cooling the ceramic powder and thewater obtained in the step of dissolving, wherein said coating consistsof said ceramic powder or said ceramic powder and an additive selectedfrom the group consisting of Nb₂ O₅, Co₃ O₄, MnO₂, Ta₂ O₅, and VO₅ ; andthe step of firing a mixture of the ceramic powder obtained in the stepof depositing at a high temperature to subject the deposited material tosolid phase reaction to form a shell portion on and around the coreportion of the ceramic powder.
 2. The process for preparing the ceramicpowder as claimed in claim 1, wherein said ceramic powder is heated withwater under elevated pressure.
 3. The process for preparing the ceramicpowder as claimed in claim 1, wherein said ceramic powder is bariumtitanate powder, barium titanate zirconate powder, lead titanatezirconate powder or lead titanate powder.